The Kroll method for reducing TiCl4 by Mg is generally used as an industrial production method of the metallic Ti. In the Kroll method, the metallic Ti is produced through a reduction step and a vacuum separation step. In the reduction step, TiCl4 which is of a raw material of Ti is reduced in a reactor vessel to produce the sponge metallic Ti. In the vacuum separation step, unreacted Mg and MgCl2 formed as a by-product are removed from the sponge metallic Ti produced in the reactor vessel.
To explain the reduction step in detail, in the reduction step, the reactor vessel is filled with the molten Mg, and the TiCl4 liquid is supplied from above a liquid surface of the molten Mg. This allows TiCl4 to be reduced by Mg in the vicinity of the liquid surface of the molten Mg to generate the particulate metallic Ti. The generated metallic Ti is sequentially sedimented downward. At the same time, the molten MgCl2 is generated as the by-product in the vicinity of the liquid surface. A specific gravity of molten MgCl2 is larger than that of the molten Mg. The molten MgCl2 which is of the by-product is sedimented downward due to the specific-gravity difference, and the molten Mg emerges in the liquid surface instead. The molten Mg is continuously supplied to the liquid surface by the specific-gravity difference substitution, and the reaction is continued.
In the metallic Ti production by the Kroll method, a high-purity product can be produced. However, in the Kroll method, because the product is produced in a batch manner, a production cost is increased and the product becomes remarkably expensive. One of factors of the increased production cost is the difficulty of enhancing a feed rate of TiCl4. The following is cited as the reason why the feed rate of TiCl4 is restricted.
In order to improve productivity in the Kroll method, it is effective to enhance the feed rate of TiCl4 which is of the raw material of Ti, i.e., to enhance a supply amount of molten Mg to the liquid surface per unit area or unit time. However, when the feed rate is excessively enhanced, the rate of the specific-gravity difference substitution cannot respond to the reaction rate, MgCl2 remains in the liquid surface, and TiCl4 is supplied to the MgCl2, which reduces utilization efficiency of TiCl4.
As a result, the supplied raw material becomes unreacted generation gas (referred to as unreacted gas) such as unreacted TiCl4 gas and unreacted TiCl3 gas, and the unreacted gas is discharged outside the reactor vessel. It is necessary to avoid the generation of the unreacted gas, because a rapid increase in inner pressure of the reactor vessel is associated with the generation of the unreacted gas. There is a limit of the feed rate of TiCl4 which is of the raw material of Ti for the above reasons.
When the feed rate of TiC4 is enhanced, a precipitation amount of Ti is increased in the inner surface of the reactor vessel above the liquid surface. As the reduction reaction proceeds, the liquid surface of the molten Mg rises intermittently. Therefore, the precipitated Ti in the inner surface of the upper portion of the reactor vessel is immersed in the molten Mg in a late stage of the reduction reaction, which causes the effective area of the Mg liquid surface to be decreased to reduce the reaction rate. In order to suppress the reduction of reaction rate, it is necessary that the feed rate of TiCl4 be restricted to prevent the Ti precipitation in the inner surface of the upper portion of the reactor vessel. Japanese Patent Application Publication No. 8-295955 proposes a countermeasure for suppressing the Ti precipitation in the inner surface of the upper portion of the reactor vessel. However, the countermeasure proposed in Japanese Patent Application Publication No. 8-295955 is not sufficient.
In the Kroll method, since the reaction is performed only in the vicinity of the liquid surface of the molten Mg solution in the reactor vessel, an exothermic area is narrowed. Therefore, when TiCl4 is supplied at a high rate, cooling cannot keep up with the supply of TiCl4 in the reaction area. This also causes the feed rate of TiCl4 to be restricted.
Although the feed rate of TiCl4 is not directly affected, in the Kroll method, Ti is generated in the particulate form in the vicinity of the liquid surface of the molten Mg solution, and Ti is sedimented. However, because of wetting properties (adhesion properties) of the molten Mg, the generated Ti particles are sedimented while aggregated, and the Ti particles is sintered to grow in particulate size of the Ti particles at a melt temperature condition during the sedimentation, which makes it difficult to retrieve the Ti particles out of the reactor vessel. Therefore, in the Kroll method, the continuous production is difficult to perform, and the improvement of the productivity is blocked. This is why the Ti is produced in the batch manner in the form of the sponge titanium by the Kroll method.
With reference to the Ti production methods except for the Kroll method, for example, U.S. Pat. No. 2,205,854 describes that, in addition to Mg, Ca can be used as the reducing agent of TiCl4. U.S. Pat. No. 4,820,339 describes a method for producing Ti through the reduction reaction by Ca, in which the molten salt of CaCl2 is held in the reactor vessel, the metallic Ca powder is supplied into the molten salt from above, Ca is dissolved in the molten salt, and the TiCl4 gas is supplied from below to react the dissolved Ca with TiCl4 in the molten salt of CaCl2.
In the reduction by Ca, the metallic Ti is generated from TiCl4 by the reaction of the following chemical formula (a), and CaCl2 is also generated as the by-product at the same time. Ca has an affinity for Cl stronger than that of Mg, and Ca is suitable for the reducing agent of TiCl4 in principle:TiCl4+2Ca→Ti+2CaCl2  (a)
Particularly, in the method described in U.S. Pat. No. 4,820,339, Ca is used while dissolved in the molten CaCl2. When the reduction reaction by Ca is utilized in the molten CaCl2, like the Kroll method, TiCl4 is supplied to the liquid surface of the reducing agent in the reactor vessel, which enlarges the reaction area compared with the case in which the reaction area is restricted in the vicinity of the liquid surface. Accordingly, because the exothermic area is also enlarged to facilitate the cooling, the feed rate of TiCl4 which is of the raw material of Ti can be largely enhanced, and the remarkable improvement of the productivity can be also expected.
However, it is difficult that the method described in U.S. Pat. No. 4,820,339 is adopted as the industrial Ti production method. In the case where the metallic Ca powder is used as the reducing agent, because the metallic Ca powder is highly expensive, the purchase and use of the metallic Ca powder leads to increase the production cost to be higher than that of the Kroll method in which the feed rate of TiCl4 is restricted. In addition, highly reactive Ca is extremely difficult to handle, which also causes the factor of blocking the industrial application of the method for producing Ti through the reduction by Ca.
U.S. Pat. No. 2,845,386 describes the Olsen method as another Ti production method. The Olsen method described in U.S. Pat. No. 2,845,386 is a kind of oxide direct-reduction method for directly reducing TiO2 by Ca. Although the oxide direct-reduction method is highly efficient, since it is necessary to use expensive high-purity TiO2, the oxide direct-reduction method is not suitable for producing the high-purity Ti.